The present invention relates generally to circuits for image pickup tubes of the electromagnetic focusing type. More particularly, it relates to a simple circuit capable of stabilizing an electron beam focusing in an electromagnetic focusing type camera tube. This stabilization utilizes the relationship between the voltage of the focusing electrode and the current in the focusing coil of the camera tube.
In general, the focusing of the electron beam of an electromagnetic focusing type camera tube is determined by the mutual relationship between the voltage of the focusing electrode and the current flowing through the focusing coil of the camera tube.
More specifically, the focusing magnetic field is created in the focusing coil by the focusing current flowing therethrough. The electric field is created by the voltage applied to the focusing electrode and has the greatest effect on the variation of the focus point (electrical focus point) of the electron beam. The other electrodes of the camera tube have only a small effect upon the focusing of the electron beam.
Accordingly, in order to attain good focusing, it has heretofore been a practice to provide a separate high voltage generating circuit for supplying voltage to parts such as the focusing electrodes and a separate circuit for supplying current to the focusing coil. This separation increases their stability. However, in order for these circuits to have high stability, a great number of expensive parts are necessary, whereby the costs and prices of these circuits become disadvantageously high.
Furthermore, if there is a fluctuation in the voltage of the power source for the high voltage generating circuit and focusing current supplying circuit, their output voltages fluctuate with the same fluctuation ratio. The focusing of the electron beam changes in response to this fluctuation of the power source voltage. Furthermore, the focusing of the electron beam is also affected by other factors. Accordingly, it has heretofore been necessary to adjust controls whenever the focusing state of the electron beam varies because of fluctuation of the power source voltage or for some other cause.
The analytical considerations leading to the present invention will now be set forth. The electrical focal distance f, when the electromagnetic focusing camera tube is in its optimum focusing state, is expressed by the following equation in terms of the above mentioned voltage V of the focusing electrodes and the magnetic flux density B of the magnetic field in the focusing coil: ##EQU1## The constant K in this Eq. (1) is expressed by the following equation in terms of the mass m of the electrons and the energy e of the electrons: EQU K=8m/e (2)
The quantity Bx in the denominator on the right-hand side of Eq.(1) can be considered to be constant because the magnetic field in the focusing coil is uniform. Accordingly, Eq.(1) can be simplified as follows: EQU f.apprxeq.K(V/B.sup.2) (3)
The magnitude of the magnetic flux density B is proportional to the magnitude of the current I flowing through the focusing coil. Accordingly, the electrical focal distance f of the optimum focusing state can be expressed as follows, in terms of the focusing electrode voltage V and the focusing current I: EQU f.varies.V/I.sup.2 ( 4)
As is apparent from Eq.(4), in an electromagnetic focusing camera tube, the position of the electrical focus point does not fluctuate as long as the quantity V/I.sup.2 is constant. Then, if there is the relationship given by the following equation between the current regulation K.sub.I of the focusing coil current I and the voltage regulation K.sub.V of the focusing electrode voltage V, the position of the electrical focus point cannot fluctuate: EQU (1+K.sub.I).sup.2 =1+K.sub.V ( 5)
A first-degree approximation of Eq.(5) produces the following equation: EQU 2K.sub.I =K.sub.V ( 6)
Therefore, it can be concluded that, in an electromagnetic focusing camera tube, substantially no fluctuation occurs in the position of the electrical focus point even when there are small fluctuations in the focusing electrode voltage V and the focusing coil current I, provided that the condition expressed by Eq.(6) is valid. Accordingly, the present invention is based on a utilization of this relationship.